NFC: Fix some darwin linker warnings introduced in r338385

The darwin linker was complaining about Toolchains/RISCV.cpp and
Toolchains/Arch/RISCV.cpp had the same name. Fix is to just rename
Toolchains/RISCV.cpp to Toolchains/RISCVToolchain.cpp.

Differential revision: https://reviews.llvm.org/D52574

llvm-svn: 343263